1. Field of the Invention
The present invention concerns metal materials of excellent corrosion resistance plated with a Zn-Mg alloy by vapor deposition, which are extremely useful as outer plates for vehicles or home electrical or electronic products, as well as various building materials. They include those materials that are used with or without coating, as well as include those materials used with or without fabrication such as in press molding.
Metal substrate materials to be plated in accordance with the present invention include Fe or Fe-based alloys, as well as non-iron metals such as Cu, Al and Ti or alloys thereof. They may be in any of shapes such as plates, corrugated, tubular or rod-like materials, as well as profiled materials H- or I-cross sectioned rods, with no particular restrictions. Although explanation is to be made hereinafter mainly for the most representative steel sheets, the technical scope of the present invention should not be construed only thereto.
2. Description of the Prior Art
Zn-plating has generally been employed as means for applying corrosion Resistant proof to steel sheets, etc. However, the demand for the improvement of the corrosion proofness of steel sheets, etc. has because increased, and Zn-plated steel sheets in the prior art can no more satisfy the requirement of users. It may be considered to improve the corrosion resistance by increasing the deposition amount of plating, but it brings about drawbacks such as powdering upon press molding, degradation upon spot welding and increased cost.
In view of the such a technical background, steel sheets plated with Zn-alloys such as Zn-Ni, Zn-Fe, Zn-Mn, Zn-Co, etc. utilizing the electroplating method have been developed, as well as steel sheets plated with multi-component Zn-Al alloys such as Zn-Al-Misch metal, Zn-Al-Si, Zn-Al-Mg, Zn-Al-Sn, etc. using the hot dip galvanizing method have also been developed.
However, for the Zn-alloy plated steel sheets obtained by the electroplating method, the following problems have been pointed out.
Zn-Ni plated steel plates:
Since the plating layer shows high hardness within a range of composition for providing corrosion resistance, cracks are developed to the plated layers when fabrication is applied to the plated steel sheets and red rusts are formed on the substrate steel sheets exposed through the cracks. Particularly, in the case of applying press molding, an undesired phenomenon such as powdering and flaking occurs in which the plating layers are peeled off from the substrate steel sheets and, accordingly, the corrosion resistance after the fabrication is remarkably low. In the case of pure Zn-plating, since the plating layers are too soft, the plating layers are deposited to the die upon press molding thereby causing pressing flaws upon pressing the succeeding works, as well as it deteriorates the corrosion resistance per se.
Zn-Fe plated steel sheet:
Since the plating layer contains Fe, red rusts are formed during use for a relatively short period of time. It also causes problems such as powdering or flaking upon pres molding like that Zn-Ni plating.
Zn-Mn plated steel sheet:
Since a great amount of hydrogen is evolved upon forming plating layers, current efficiency is lowered to bring about a problem with respect to productivity.
Zn-Co plated steel sheet:
Plating layers is extremely difficult in the fabrication like that of the Zn-Ni plated steel sheet and, when fabrication is applied to the plated steel sheet, cracks, are developed to the plating layer to form rusts through the crackings.
Next, in the case of steel sheets plated with Zn-Al series multi-component alloy described above obtained by the hot dip galvanizing method, since the alloying element (or elements) combined have to be compatible with molten Zn, the base metal is limited only to the Zn-Al series, as well as there are restrictions to the kind and the blending amount of other alloying elements, thereby failing to obtain improving effect of the corrosion resistance as expected. In addition, the plating layer also involves a problem that has two phases, i.e., Zn-enriched phase and Al-enriched phase are present together in admixture and the grain boundary between both of the phases is liable to be corroded. Further, in the hot dip galvanizing method, if the temperature for a plating bath is too high, there also occurs a problem that a brittle intermetallic compound layer containing Fe is formed at the boundary between the plating layer and the substrate steel sheet, thereby easily peeling off the plating layer upon fabrication. In addition, for the conventional Zn series plating, there has been noted a common defect that the corrosion resistance after the coating is not always satisfactory.
As has been described above, conventional Zn series plated steel sheets involve problems in the corrosion resistance, fabricability, productivity, etc. and development has been sought for Zn series plated steel sheets capable of satisfying these requirements altogether.
In view of the situations as described above, various studies have been made for the chemical treatment in the Zn-series plating. The most fundamental method for the chemical treatment, there can be mentioned a chromate treatment. However, film layers formed by the conventional chromate treatment are extremely thin. Accordingly, Zn-series plating layers on the substrate are liable to be exposed due to the flaws formed in the chromate film layers or the defoliation of such layers, etc. and, accordingly, the corrosion resistance of the Zn-series plated steel sheets applied with chromate treatment is not satisfactory.
Antirust sheets usually referred to as zinchrometal used as outer plates for vehicles comprise a first layer containing a chromium compound formed on a steel sheet and a second layer comprising zinc powder and resin prepared by coating zinc-enriched paints. In this case, improvement of corrosion resistance is intended by suppressing the corrosion of the substrate steel sheet by the sacrificial protective effect of Zn powder in the zinc-enriched paint layer of the second layer and, thus, electro-depositing coating and spot welding are enabled by utilizing the electroconductivity. However, for attaining sufficient corrosion resistance electrodeposition coatability and spot weldability in the steel sheets described above, it is necessary to increase the content of the zinc powder in the zinc-enriched paint to more than 80%. However, if the Zn content is so increased, it brings about a problem wherein the zinc-enriched paint layer is peeled off in a powdery form (flaking phenomenon) upon applying press molding, etc. to deteriorate the fabricability. Accordingly, in order to maintain a sufficient fabricability, the content of the Zn powder may be decreased. However, if the content of the Zn powder is decreased, it causes the reduction in the corrosion resistance, as well as reduction in the electrodeposition coatability and the spot weldability due to the lowering in the electroconductivity.
Further, Japanese Patent Laid-Open Nos. Sho 58-98172 and 57-108292 disclose composite coated steel sheets in which a chromate film layer is formed on a Zn series and/or Al series plating layer and a composite organic silicate film is formed further thereover. Although anti-rust steel sheets disclosed in these laid-open publications can satisfy the fabricability, as well as show excellent properties in view of corrosion resistance, adherance and corrosion resistance of paints, etc., there are still left problems in view of the corrosion resistance in close examination and, further, there has been an increasing demand for antirust steel sheets having more excellent corrosion resistance.
Zn-Mg alloy plating has been developed in view of the foregoing demand, which has been produced so far by means of a hot dip galvanizing method. Zn-Mg alloy plating by hot dip galvanizing method can provide some effect for the corrosion resistance, but it involves the following problems. That is, a top-dross is formed much on much of the surface of molten Zn-Mg bath. This is the reason why molten Mg is oxidated easily in the air. The formation of the top-dross causes the poor appearance of the plated steel sheet and the reduction of the amount of molten Zn-Mg alloy which can use for plating. They are very disadvantageous in view of the manufacturing of hot dip galvanized steel sheets.
The hot dip galvanizing method shows a further problem as described below. That is, since the melting point of Mg (651.degree. C.) is much higher as compared with that of Zn (419.degree. C.), there is a limit for the amount of Mg that can be added to molten Zn in the case of the hot dip galvanizing, and it is impossible to optionally control the plating composition of Zn-Mg alloys. Accordingly, since Mg can not be added by a desired amount, Mg addition effect can not always be obtained sufficiently. In addition, since the hot dip galvanizing process is conducted under high temperature condition, oxide layers or intermetallic compound layers are formed between the plating layer and the steel sheet to obtain only those plating products lacking in good adhesion and fabricability. Further, there is also a problems that the plated substrate such as steel sheets are denatured by the effect of heat treatment (formation of heat treated texture).
Furthermore, if it is intended to employ an electroplating method using usual aqueous solution, since the oxidation--reduction potential of Mg is less noble in comparison with that of hydrogen, electrodeposition of Mg is impossible.